I'm working on my third project on PCB and my bootloader won't load to an 32U4, using an Arduino Uno as ISP.
Probably I messed up a part of the circuitry on my PCB, but the 32U4 documentation doesn't immediatly point to something obvious. Maybe the feedback of the bootloader could tell me something useful?
I'm looking through the verbose feedback, but trying to google what all this actually means, it seems like the actual documentation is hard to find (just some nice but prozaic tutorials and loads of reminders to check your wiring). Can anyone help me?
Usually bootloaders are very stripped down and don't provide useful error codes. That's why there are so many answers to the "why do I get this "out of sync" error?" question - the PC sent a command where it expected to receive a "sync" response,a nd it didn't get one. That can mean any number of things - wrong port, wrong speed, wrong software (board-type), wrong wiring, etc.
The VERY verbose debugging from avrdude will show every byte that is sent or received, and if you've read up on the protocols use, you MIGHT be able to recognize something "interesting" that leads to a solution.
Instead of using that link, I would recommend downloading the same documentation from the official AVRDUDE website: http://download.savannah.gnu.org/releases/avrdude/
The reason is that you can find the documentation for the version of avrdude that you're using. The documentation at your link is for avrdude 5.1, but Arduino is now using avrdude 6.3.